Hybrid wireless cellular communications devices (WCDs) are capable of communicating on both cellular networks and in broadband wireless networks, such as, 802.11 protocol-based or WLAN-based networks. As the WCD moves physically and/or the fading channel changes due to subtle variations in the complexity of the physical surroundings, the WCD supports a specific set of logical decision-making capabilities which determine how a cell and/or network will be selected. Generally, a hybrid WCD may detect and select one network or the other, or both.
Broadband wireless communication protocols support radio resource management techniques for detecting one or more operating frequencies and access points. A cellular system, such as Global System for Mobile telecommunication (GSM), however, has little in common with alternate radio access interfaces, for example, a standardized WLAN like 802.11 or other wireless technologies which are capable of operating over unlicensed spectrum. The differences in radio behavior result primarily from differences in operating bandwidths, power limitations for unlicensed operation, Medium Access Control (MAC) protocols designed to handle different predominant traffic types, frequency ranges of operation and radio propagation characteristics for licensed/unlicensed operation.
When WCD moves from a cellular network operating at one radio frequency (RF) to another network (e.g., WLAN) operating at another radio frequency (RF), or vice-versa, it is often necessary for the WCD to undergo a hard handover (or “handoff”) from the cellular network to the other network, or vice-versa. There are a number of inter-system (or inter-frequency) handover techniques for making this happen.
Handover performance can be analyzed in terms of variables such as unnecessary handovers and processing delay time in making a handover decision. It is desirable to reduce both of these variables. A short signal averaging time may result in an increase in unnecessary handovers, while a long averaging time may result in a failure to detect a necessary handover. In most, if not all, inter-system handover techniques, the processing delay and stability are important considerations which can affect system performance.
To address these issues, an averaging window (AW) can be utilized to accumulate a certain number of Channel Quality Measurement (CQM) samples over a given time frame. The CQM samples are then averaged to provide an estimate of the current CQM. For example, according to one approach, a current estimated channel quality measurement/metric (CQM) is calculated by using a real-time window to obtain previous actual CQM samples, P(i), and then averaging the previous actual CQM samples P(i) to obtain current estimated CQM. The number of CQM samples needed to make a reasonable estimate of the current CQM varies depending on the system. However, regardless of the system, it takes a certain amount of time to accumulate the CQM samples. The time required to accumulate the CQM samples introduces some delay into estimating the current CQM. This delay can be referred to as “averaging” or “accumulation” delay. This “averaging” delay can slow down a handover decision making process and possibly disrupt the service. Thus, with such handover techniques, there is a tradeoff between the number of CQM samples needed to accurately reflect the average performance of the system and the time required to accumulate the CQM samples.
Introduction of a hysteresis level can improve the stability of the handover and help ensure that necessary handover occurs, while the unnecessary handovers are reduced. This helps to reduce “ping-pong” type handovers. However, application of the hysteresis level results in a “hysteresis” delay in making the handover decision which can impact the speed of the handover. Thus, it is desirable to keep this “hysteresis” delay as small as possible.
Notwithstanding these advances, it would be desirable to further reduce and/or eliminate the effects of averaging delay and hysteresis delay when making a handover decision. Other features and characteristics of the present disclosure will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.